This invention relates to a biological filter for the purification of waste gases and to the purification of air or gas streams containing odorous or toxic compounds by passage through a porous solid media containing micro-organisms which convert the contaminant compounds to oxidised products, thereby effecting the desired purification. This invention, more particularly, relates to passing contaminated air through a bioactive biofilter media comprising retted coconut pith and coconut fibre, and to the selection, preparation and arrangement of the media. The invention achieves superior performance as a biofilter by enabling the adjustment of the pressure drop to a desired value, by providing adequate water retention, by providing a conducive environment for microbial growth, by using constituents which are natural, stable and environmentally compatible, by reducing the cost of media media, and by reducing the cost of transportation media.
Biofilter systems are used in the purification of air streams. Biofilter systems have been applied for removal of odorous and toxic compounds in a wide variety of situation (Ottengraph S.S.P., Exhaust gas purification, Chapter 12, Biotechnology Vol. 8, Verlagsgesellschafi, Wienheim, 1986: VDI Guidelines No: 3477 xe2x80x94xe2x80x9cBiofilterxe2x80x9d, Verein Deutscher Ingenieure, Dusseldorf 1991). During the passage of air through the biofilter, micro-organisms such as bacteria and fungi which are active on the biofilter media, oxidise the toxic and odour substances present in the air stream to carbon dioxide water and mineral products such as mineral acids depending on the chemical constitution of the contaminant. The biofilter media is a critical component of the system providing physical support, moisture and nutrients for the microorganisms while ensuring sufficient contact with gases at low pressure drop. In this invention, the removal of odour and toxics from air is achieved by passing air through a biofilter comprising retted coconut pith and coir fibre.
In general, two types of materials have been used in biofilter processesxe2x80x94natural materials such as biocompost, peat, fibrous peat, and porous clays or synthetic materials such as plastics. The use of natural organic materials usually provides the nutrients required to support growth of micro-organisms. The use of synthetic materials requires continuous addition of nutrients but allows better control of operation by facilitating the removal of acidity and degradation products. This invention falls in the category of natural material biofilters and one aspect of this invention relates to the selection, preparation and arrangement of the bed. There are only a few disclosures in the prior art regarding the selection and preparation of biofilter carrier material and formation of a biofilter bed.
Reference may be made to U.S. Pat. No. 5,578,114 dated Nov. 26, 1996, granted on Oude Luttighuis wherein a biological filter with a palletized biologically active compost carrier material is disclosed. Here, compost material is formed into balls 1 to 30 mm diameter, using water and thereafter drying the pellets. The object of the invention seems to be to reduce pressure drop. While this object is achieved, the invention fails to provide intimate gas contact with the whole of the carrier, as gas will pass through the space between the balls. Thus, the invention adversely affects the main object of a biofilter because the reaction between gas and active carrier media will be retarded as a result of poor mass transfer. This is also evident from the rather poor elimination capacity of ethyl acetate contaminant, in the example quoted therein, particularly and simpler to degrade than other common volatile organic compounds such as toluene, the removal of which is a common biofilter application.
It is disclosed in EP0142872 dated Jan. 27, 1988, that the addition of inert materials such as polyethylene, polystyrene, ground automobile tyres, volcanic lava, perlite and such inert materials, with a diameter of 3 to 10 mm can reduce the resistance to flow of gas through a biofilter compost bed. These materials are claimed to prevent shrinkage cracks as a result of differential drying that could take place in the bed. The claim appears to be based on the rigidity and inertness of the additional material added. The proportion of this inert material is 50% to 70% by volume. It is noted that these materials, being rigid, substantially spherical (certainly not fibrous), non-porous and water repelling are thus generally non-active. They support very low numbers of microorganisms and therefore reduce the useful volume of the biofilter.
Thus, the elimination capacity per unit volume of a biofilter is reduced by the volume of the inert additives. In practice, it is seen that mainly polystyrene balls are used as additive, because of its low weight, in commercial applications provided by the assignee of the above patent (Reference: http://www.clairtech.com). It is noted further, that the large volume of polystyrene added to compost makes the spent biofilter media environmentally unacceptable, and incurs disposal costs.
EP 413658B1 Beerli. Marc; Dec. 7, 1994; discloses a method for the preparation of peat beads which are applied to biofilters wherein the carrier material is pelletised with humidified turf as a binder. This again suffers the same disadvantages as with U.S. Pat. No. 5,578,114.
The main object of the present invention to provide an environmentally friendly inexpensive and highly effective biofilter for deodourisation and detoxification of air.
It is another object of the present invention to provide a biofilter for deodourisation of air utilizing biofilter system where pressure loss during passage of air is low and where the pressure loss may be adjusted as required by the addition of a bulking agent without compromising the elimination capacity per unit volume of the biofilter.
It is yet another object of this invention to provide a biofilter for deodourisation of air, which is able to retain and provide large moisture content for support of growth of micro-organisms.
It is yet another object of this invention to develop a new application for an agrowaste which, at present, incurs disposal costs.
All the above objects are achieved by the formulation of a biofilter bed using retted coconut pith as biocarrier and coir fibre as bulking agent.
Still another object of this invention to provide a biofilter process for deodourisation of air which is able to sustain activity for a long duration without compaction, ageing and pressure build-up. This object is achieved by passing contaminated air through a bed comprising retted coconut pith and if required, coir fibre, the bed providing support for microbial cultures.
Accordingly the present invention relates to a biological filter for the purification of waste gases, comprising a bed of biologically active carrier material consisting of pith extracted from coconut husks, the said bed contained such that the waste gases may be bought in contact with the said bed and removed from the bed at points spatially separated from the inlet points.
In one embodiment of the invention, the bed of biological material further comprises fibre, whereby the bed pressure drop during passage of gas is reduced.
In another embodiment of the invention, the bed of biological material further comprises acid neutralizing agents, such as calcium carbonate.
In another embodiment of the invention, the bed further comprises nitrogenous fertilizer materials.
In another embodiment of the invention, the content of the said fibre in the bed is between 0.5 to 20% weight of the moist bed.
In another embodiment of the invention, the bed of biologically active material is arranged as layers of pith extracted from coconut husk and layers of fibre, alternately.
In another embodiment of the invention, the first layer at bottom of the bed of biologically active material is a layer of fibre.
In another embodiment of the invention, the first layer at top of the bed of biologically active material is a layer of fibre.
In a further embodiment of the invention, the proportion of fibre to pith increases from the gas exhaust point to the gas inlet point.
In another embodiment of the invention, layers of fibre are constructed from non-woven fibre mat such as needle felt.
In another embodiment of the invention, the carrier material further contains activated sludge from an effluent treatment facility.
In a further embodiment of the invention, the pith is extracted from coconut husks which have undergone a process of retting.
In another embodiment of the invention, the fibre is coconut fibre.
The invention also relates to a process for the purification of a gaseous streams containing oxygen comprising humidifying the gas, if required, and passing the gas through a bed of coconut pith.
In another embodiment of the invention, the coconut pith is selected from pith extracted from coconut husks which have undergone a retting process during the process of coir manufacture.
In another embodiment of the invention, the bed further comprises fibre, such as coir.
In another embodiment of the invention, the content of fibre is between 1% and 20% the total weight of bed saturated with moisture.
The invention also relates to a process for the removal of hydrogen sulphide from air streams, air stream through a bed of retted coconut pith and coir fiber from an inlet section to an outlet section where the fibre content of the bed is high at the inlet section and low at the outlet section.